1. Field of the Invention
The present invention relates to a motor-driven power steering apparatus for a vehicle.
2. Description of the Related Art
In a motor-driven power steering apparatus for a vehicle, there is a structure made to transmit steering torque applied to a steering wheel to a pinion shaft of a rack and pinion mechanism, to transmit an auxiliary torque generated by an electric motor to the pinion shaft via a worm gear type speed reduction mechanism in correspondence to the steering torque, and to steer a steered wheel by the rack and pinion mechanism, as described in Japanese Unexamined Patent Publication No. 2001-278077 (patent document 1).
In the motor-driven power steering apparatus for the vehicle described above, there is proposed a structure in which a reaction force from a road surface is transmitted to the worm gear type speed reduction mechanism via the rack and pinion mechanism. In order to lower vibration which may cause a beating sound generated due to play between a worm wheel and a worm constituting the worm gear type speed reduction mechanism, an elastic body is interposed in an auxiliary torque transmission path of the worm gear type speed reduction mechanism and the rack and pinion mechanism.
FIG. 8A shows a conventional motor-driven power steering apparatus for a vehicle having a right-hand drive specification. Reference numeral 1 denotes a pinion shaft, reference numeral 2 denotes a rack engaging with a helical pinion 1A of the pinion shaft 1, reference numeral 3 denotes a worm wheel fixed to the pinion shaft 1, and reference numeral 4 denotes a worm engaging with the worm wheel 3. The direction of a torsion angle β of the helical pinion 1A provided in the pinion shaft 1 is set opposite to a direction of a tooth lead angle γ of the worm wheel 3. In FIG. 8A, the pinion shaft 1 and the worm wheel 3 are displaced in a direction B and are rotated in a direction C, and the worm 4 is displaced in a direction D due to a road surface reaction force A applied to the rack 2 during straight running of the vehicle. At this time, the directions B and D are opposite directions, and the worm wheel 3 and the worm 4 are displaced in a direction which reduces the play that exists between the worm wheel 3 and the worm 4 before being displaced, so that the vibration of the worm gear type speed reduction mechanism caused by the reaction force from the road surface is not amplified.
Alternatively, FIG. 8B shows a conventional motor-driven power steering apparatus for a vehicle having a left-hand drive specification. Reference numeral 1 denotes a pinion shaft, reference numeral 2 denotes a rack engagable with a helical pinion 1A of the pinion shaft 1, reference numeral 3 denotes a worm wheel fixed to the pinion shaft 1, and reference numeral 4 denotes a worm engaging with the worm wheel 3. The torsion angle β of the helical pinion 1A provided in the pinion shaft 1 is set to the same direction as the direction of a tooth lead angle γ of the worm wheel 3. In FIG. 8B, the pinion shaft 1 and the worm wheel 3 are displaced in a direction B and are rotated in a direction C. Worm 4 is displaced in a direction D due to a road surface reaction force A applied to the rack 2 during a straight running of the vehicle. At this time, the directions B and D are same, and the worm wheel 3 and the worm 4 are displaced in a direction that increases the play existing between the worm wheel 3 and the worm 4 before being displaced, so that the vibration of the worm gear type speed reduction mechanism caused by the reaction force from the road surface is amplified.
In this situation, the elastic body quality deteriorates with time and can not maintain a stable vibration proofing performance, even when the elastic body in the patent document 1 is used in a vehicle having the left-hand specification of FIG. 8B. Further, this construction requires an increased number of parts.